Eddy current testing is a technique that can be used to determine the presence of flaws, such as cracks, in a specimen composed of conductive materials. Eddy current testing utilizes electromagnetic induction, where a coil of a probe is placed proximate to a test specimen that is formed from conductive materials. The coil is energized via a current to create a magnetic field. The magnetic field induces eddy currents in the conductive materials of the test specimen, which generate a secondary magnetic field. The nature of the secondary magnetic field, such as its magnitude or directionality, at least partially depends on the structural features of the test specimen. For example, cracks, dents, or other structural irregularities may induce perturbations in the secondary magnetic field.
The secondary magnetic field is also affected by probe liftoff and changes in the material properties of the test specimen, for example magnetic permeability, electrical conductivity, and residual magnetization. These probe liftoff and material property variations, if unaccounted for, can result in false positive or false negative indications of wall loss.